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Yersinia pestis
'' micrograph depicting a mass of Y. pestis bacteria in the foregut of an infected flea]]Yersinia pestis'' (formerly Pasteurella pestis) is a Gram-negative, non-motile, rod-shaped, coccobacillus bacterium, with no spores. It is a facultative anaerobic organism that can infect humans via the oriental rat flea. It causes the disease plague, which takes three main forms: pneumonic, septicemic, and bubonic plagues. All three forms were responsible for a number of high-mortality epidemics throughout human history, including the sixth century's Plague of Justinian; the Black Death, which accounted for the death of at least one-third of the European population between 1347 and 1353; the Great Plague of London of 1665, which was ended in 1666 by the Great Fire of London; and the Third Pandemic, sometimes referred to as the Modern Plague, which began in the late 19th century in China and spread by rats on steamboats, claiming close to 10 million lives. These plagues likely originated in Chima and were transmitted west via trade routes. Recent research in 2018 indicated that the pathogen may have been the cause of what was described as the Neolithic decline, in which European populations declined significantly. This would push the date to much earlier and might be indicative of an origin in Europe rather than Eurasia. Y. pestis was discovered in 1894 by Alexandre Yersin, a Swiss/French physician and bacteriologist from the Pasteur Institute, during an epidemic of the plague in Hong Kong. Yersin was a member of the Pasteur school of thought. Kitasato Shibasaburō, a German-trained Japanese bacteriologist who practiced Koch's methodology, was also engaged at the time in finding the causative agent of the plague. However, Yersin actually linked plague with Y. pestis. Named Pasteurella pestis in the past, the organism was renamed Yersinia pestis in 1944. Every year, thousands of cases of the plague are still reported to the World Health Organization, although with proper treatment, the prognosis for victims is now much better. A five- to six-fold increase in cases occurred in Asia during the time of the Vietnam War, possibly due to the disruption of ecosystems and closer proximity between people and animals. The plague is now commonly found in sub-Saharan Africa and Madagascar, areas which now account for over 95% of reported cases. The plague also has a detrimental effect on non-human mammals. In the United States, mammals such as the black-tailed prairie dog and the endangered black-footed ferret are under threat. Scientific classification General characteristics Y. pestis is a non-motile, rod-shaped, facultative anaerobic bacterium with bipolar staining (giving it a safety pin appearance) that produces an antiphagocytic slime layer. Similar to other Yersinia species, it tests negative for urease, lactose fermentation, and indole. Its closest relative is the gastrointestinal pathogen Yersinia pseudotuberculosis and more distantly Yersinia enterocolitica. Pathogenesis and immunity In the urban and sylvatic cycles of Y. pestis, most of the spreading occurs between rodents and fleas. In the sylvatic cycle, the rodent is wild, but in the urban cycle, the rodent is primarily the brown rat. In addition, Y. pestis can spread from the urban environment and back. Transmission to humans is usually through the bite of infected fleas. If the disease has progressed to the pneumonic form, humans can spread the bacterium to others by coughing, vomiting, and possibly sneezing. Immunity A formalin-inactivated vaccine was once available in the United States for adults at high risk of contracting the plague until removal from the market by the Food and Drug Administration. It was of limited effectiveness and could cause severe inflammation. Experiments with genetic engineering of a vaccine based on F1 and V antigens are underway and show promise. However, bacteria lacking antigen F1 are still virulent, and the V antigens are sufficiently variable such that vaccines composed of these antigens may not be fully protective. Isolation and identification In 1894, two bacteriologists, Alexandre Yersin of Switzerland and Kitasato Shibasaburō of Japan, independently isolated in Hong Kong the bacterium responsible for the Third Pandemic. Though both investigators reported their findings, a series of confusing and contradictory statements by Shibasaburō eventually led to the acceptance of Yersin as the primary discoverer of the organism. Yersin named it Pasteurella pestis in honor of the Pasteur Institute, where he worked. In 1967, it was moved to a new genus and renamed Yersinia pestis in his honor. Yersin also noted that rats were affected by plague not only during plague epidemics, but also often preceding such epidemics in humans and that plague was regarded by many locals as a disease of rats; villagers in China and India asserted that when large numbers of rats were found dead, plague outbreaks soon followed. In 1898, French scientist Paul-Louis Simond (who had also come to China to battle the Third Pandemic) established the rat-flea vector that drives the disease. He had noted that persons who became ill did not have to be in close contact with each other to acquire the disease. In Yunnan, China, inhabitants would flee from their homes as soon as they saw dead rats, and on the island of Formosa in Taiwan, residents considered the handling of dead rats to heighten the risks of developing plague. These observations led him to suspect that the flea might be an intermediary factor in the transmission of plague, since people acquired plague only if they were in contact with recently dead rats that had died less than 24 hours before. Recent events In 2008, the plague was commonly found in sub-Saharan Africa and Madagascar, areas which accounted for over 95% of the reported cases. In September 2009, the death of Malcolm Casadaban, a molecular genetics professor at the University of Chicago, was linked to his work on a weakened laboratory strain of Y. pestis. Hemochromatosis was hypothesized to be a predisposing factor in Casadaban's death from this attenuated strain used for research. In 2010, researchers in Germany definitely established, using PCR evidence from samples obtained from Black Death victims, that Y. pestis was the cause of the medieval Black Death. In 2011, the first genome of Y. pestis isolated from Black Death victims was published and concluded that this medieval strain was ancestral to most modern forms of Y. pestis. On 8 September 2016, the Y. pestis bacterium was identified from DNA in teeth found at a Crossrail building site in London. The human remains were found to be victims of the Great Plague of London, which lasted from 1665 to 1666. Two cases of pneumonic plague were diagnosed at a hospital in Beijing's Chaoyang district on 13 November 2019, prompting fears of an outbreak. Doctors diagnosed a middle-aged man with fever, who had complained of difficulty breathing for some ten days, accompanied by his wife with similar symptoms. Police quarantined the emergency room at the hospital and controls were placed on Chinese news aggregators. On the 18th, a third case was reported in a 55-year-old male. The patient received treatment and 28 symptomless contacts were placed in quarantine. Category:Bacteria